1. Field of the Invention
The invention relates to a self-tapping screw, particularly for use in hard materials, such as bricks, concrete and rock.
2. Prior Art
A conventional self-tapping screw resembles a threadforming tap in that the threads thereof at one or more places circumferentially are cut off transversely to form sharp cutting edges which cut threads in the workpiece in which the screw is applied. Normally, screws of this type are used in sheet metal, plastic and to a certain extent in light weight concrete.
Further, screws having a generally triangular cross-section have been proposed. On such screws each turn of the thread forms three radially extending lobes between which the thread turn runs comparatively straight. The intention is that the lobes shall form threads in the workpiece, in which the screw is applied, by a plastic deformation. Also screws of this type are normally used in sheet metal, plastic and, to a certain extent, in light weight concrete.
Thread cutting screws of the first mentioned type can, for obvious reasons, not be used in hard materials, such as bricks, concrete and rock; a thread cutting in such materials is not possible. Neither are the plastically deforming, self-tapping screws useful in such hard materials.